Summary

Most machines work by moving rigid elements or frameworks. In some cases, flexible elements, such as membranes, are attached to the framework and are passively deformed by its motion. In contrast, octopuses, hearts, caterpillars, and growing mushrooms are examples of natural structures made of soft tissue that move or change configurations through a completely different mechanism. Each element of the tissue—cells or compartments—undergoes some sort of active deformation, such as swelling, that creates stresses within the structure. The entire structure changes shape to relieve these internal stresses. Such control of shape and motion by active deformation has not yet been implemented into engineering design technique because of theoretical and experimental difficulties. On page 1201 of this issue, Kim et al. (1) present a technique for the production of gel sheets that are patterned into regions that can swell to different extents and actively deform into three-dimensional (3D) shapes.